1. Field of the Invention
The present invention relates to a liquid crystal display (hereinafter referred to as an LCD), in particular to an in-plane switching (hereinafter referred to as an IPS) LCD and a manufacturing method thereof.
2. Description of the Related Art
Liquid crystal displays are one of the most highlighted flat panel displays, which use electro-optical properties of liquid crystal materials. In view of driving method, the LCDs are classified into simple matrix type and active matrix type.
An active matrix type LCD has a plurality of switching elements having non-linear characteristics, and pixels of the LCD are controlled by the switching elements. Examples of the switching elements are three-terminal thin film transistors (hereinafter referred to as TFTs) and two-terminal thin film diodes such as metal-insulator-metal (MIM) devices.
A commonly used TFT LCD is comprised of a substrate having pixel electrodes, an opposite substrate having a common electrode, and a liquid crystal material therebetween. When the pixel electrodes and the common electrode are applied with voltages, the molecules of the liquid crystal material change their orientations in response to the electric field due to the potential difference between the pixel electrode and the common electrode. Furthermore, a twisted-nematic (herein after referred to as a TN) mode where the molecular director of the liquid crystal is twisted on going from one substrate to the other substrate is generally used.
However, the LCD, in particular the TN LCD, has a narrow viewing angle and a contrast dependent on the viewing angle. In addition, it has a problem that the number of the process steps are too many since the electrodes are formed in each substrate and the two substrates have a short point for applying voltages into the common electrode.
In order to overcome these problems, IPS LCDs are suggested.
An IPS LCD has pixel and common electrodes formed in only one of the two substrates, and the voltage difference of the pixel electrodes and the common electrode produces substantially horizontal electric fields. One of the IPS LCDs is disclosed in European patent application No. 93307154.0.
Now, conventional IPS LCDs are described with reference to attached figures.
FIG. 1 and 2 are layout views of IPS LCDs disclosed in the paper entitled "Development of Super-TFT-LCDs with In-Plane Switching Display Mode" (M. Ohta et al.) of ASIA DISPLAY '95 pp. 707-710.
Fist, an LCD shown in FIG. 1 is described.
A gate line 1 is formed on a glass substrate (not shown) in a transverse direction, and a data line 11 crossing the gate line 1 is formed in a longitudinal direction. A common electrode line 2 made of the same material as the gate line 1 is arranged so as to be parallel to the gate line 1. A longitudinal branch 3 of the common electrode line 2 goes towards the gate line 1 from the common line electrode 2, and its transverse branch 4 connected to the end of the longitudinal branch 3 is arranged so as to be parallel to the gate line 1. Near the cross point of the gate line 1 and the data line 11, a TFT is formed. A gate electrode 5 and a source electrode 13 of the TFT are a portion of the gate line 1 and of the data line 11, respectively, and a drain electrode 12 of the TFT is opposite the source electrode 13. The drain electrode 12 is made of the same material as the data line 11, and there is a semiconductor layer 20 between the gate electrode 5 and the source and the drain electrodes 13 and 12. The drain electrode 12 extends to form a pixel electrode 16, 17, 18 and 19. The pixel electrode 16 has a rectangular shape having two longitudinal sides 16 and 17 and two transverse sides 18 and 19. The transverse sides 18 and 19 are overlapped with the common electrode line 2 and its transverse branch 4, respectively. These overlapped structures function as storage capacitors. The longitudinal branch 3 of the common electrode line 2 is arranged between the two longitudinal sides 16 and 17 of the pixel electrode, and the potential difference between the longitudinal sides 16 and 17 and the longitudinal branch controls performance of the liquid crystal molecules.
Next, an LCD shown in FIG. 2 is described, where the shape of the branches of the common electrode line is replaced with the shape of a pixel electrode in comparison with the LCD shown in FIG. 1.
A common electrode line 2 has two longitudinal branches 6 and 7 and one transverse branch 8 connecting the longitudinal branches 6 and 7, and thus the common electrode line 2 along with its branches 6, 7 and 8 forms a rectangle. On the other hand, a pixel electrode 13, 14 and 15 extended from a drain electrode 12 is comprised of two transverse portions 14 and 15 and a longitudinal portion 13 connecting the midpoints of the transverse portions 14 and 15. The transverse portions 14 and 15 are overlapped with the common electrode line 2 and its transverse branch 8, and these overlapped structures serve as storage capacitors. The longitudinal portion 13 is arranged between the two longitudinal branches 6 and 7 of the common electrode line 2, and the potential difference between the longitudinal branches 16 and 17 and the longitudinal portion 13 adjusts performance of the liquid crystal molecules.
It is disclosed in the above-described paper that these LCDs have improved viewing characteristics.
However, the paper lodges some issues.
First, the liquid crystal molecules should be shielded from the electric fields produced from the data line.
The longitudinal side 16 of the pixel electrode shown in FIG. 1 and the longitudinal branch 6 of the common electrode shown in FIG. 2, both adjacent to the data line 11, serve as shields from the electric field of the data line 11. Since the pixel electrode is floated when the TFT is in off state, its potential is easily disturbed by the potential of outside. On the other hand, the common electrode line and its branches are not easily disturbed since they are constantly applied with voltages.
Accordingly, the LCD shown in FIG. 2 effectively shields the electric fields from the data line.
Second, aperture ratio should be considered since all the electrodes are formed in a substrate.
The aperture ratio is determined by the effective display area, that is, the area surrounded by the pixel electrodes 16, 17, 18 and 19 shown in FIG. 1 or the area enclosed by the common electrode line 2 and its branches 6, 7 and 8 shown in FIG. 2. In case of the LCD shown in FIG. 1, the longitudinal side 16 of the pixel electrode adjacent to the data line 11 should be separated from the data line 11 by predetermined distance in order to prevent their short-circuiting since the pixel electrode 16, 17, 18 and 19 and the data line 11 are made of the same layer. On the other hand, concerning the LCD shown in FIG. 2, since the common electrode line 2 and its branches 6, 7 and 8 are insulated from the data line 11 via an insulating layer (not shown), it is permissible that they become considerably closer.
Therefore, the LCD shown in FIG. 2 is effective to obtain large aperture ratio.
However, these conventional IPS LCDs have a disadvantage that they cannot sufficiently shield the electric fields from the data line. Further, there are disadvantages that the capacitance of a parasitic capacitor comprised of a gate and a source of a TFT is not uniform between pixels when misalign occurs, and the on current of the TFT is low.